Inconsistent stats in v$filestat

Can someone explain why the avgiotim column from v$filestat doesn't match the time computed by dividing total i/o's by total i/o time? Timed statistics is set to true. The platform is AIX 4.3, Oracle 8.1.7.

col avgiotim heading "AVG|IO|TIME"
col tot_io_time format 99999999999 heading "TOT|IO|TIME" col computed_avg_io_time heading 'COMPUTED|AVG IO|TIME'

SELECT FILE#
, avgiotim
, phyrds + phywrts tot_physio
, readtim + writetim tot_io_time
, (readtim + writetim) / (phyrds + phywrts) computed_avg_io_time
FROM v$filestat
WHERE file# in (4,14)
ORDER BY 3 DESC
/

              AVG                     TOT   COMPUTED
               IO                      IO     AVG IO
 FILE#       TIME TOT_PHYSIO         TIME       TIME
------ ---------- ---------- ------------ ----------
    14          0  117405621  10124912412 86.2387365
     4          4        320          422    1.31875